This section provides background information related to the present disclosure which is not necessarily prior art.
Optical switches have been pursued for compact, high voltage, high current, short pulse switching applications since the early 1970's. Switch development has focused on using semiconductor materials as photoconductors. Charge carriers are generated in the semiconductor material by applying optical excitation with photon energies that are greater than, or less than, the bandgap of the semiconductor.
With intrinsic photoconductive switches, electron-hole pairs are generated when a semiconductor is illuminated with above bandgap light. An optical transition that generates an electron-hole pair is shown in the simplified band diagram of FIG. 1. A photon with energy, ℏω, greater than or equal to the bandgap (Eg), will be absorbed and excite an electron from the valence band to the conduction band, leaving a hole in the valence band. The number of charge carriers generated in the semiconductor material is proportional to the number of photons absorbed. Unfortunately, above bandgap light is rapidly absorbed in the material leading to a shallow optical penetration depth (on the order of 10's of microns). As a result, the switch conductivity is high in a thin layer at the surface of the switch material.
Photoconductive switches excited by photons with energy greater than the switch material bandgap are usually constructed in the lateral geometry of FIG. 2. FIG. 2 shows the optical excitation being applied to the surface of a semi-insulating semiconductor between a switch anode metallization layer and a cathode metallization layer, which form the two electrodes for the switch. The shallow penetration of light requires the switch electrodes to be positioned on the same surface of a switch substrate that is thicker than the optical penetration depth. The ultimate performance of a lateral switch, such as shown in FIG. 2, is limited by surface flashover and surface recombination. However, switches using above bandgap light have the advantage that the switch conductivity will be a linear function of applied optical power and there will be no saturation of carrier density due to the wealth of electron (holes) residing in the conduction (valence) band of the switch. There are also nonlinear versions of the lateral switch constructed from GaAs that use low intensity, above bandgap optical excitation to generate a small number of electron-hole pairs that induce an avalanche breakdown in the switch material termed “Lock-on”.
Extrinsic photoconductive switches are switches in which photons with energy less than the switch material bandgap can excite electrons (holes) from energy levels located in the bandgap into the conduction band (valence band). This type of photoconductive switch is called an “extrinsic” switch since carrier generation is dependent on the density of impurity and defect energy levels in the switch material. The operating principle of an extrinsic photoconductive switch is shown in FIG. 3. Photons with energy ℏω<Eg/2 can excite electrons (holes) trapped in acceptor (donor) levels into the conduction band. In addition, photons with energy ≥Eg/2 generate electrons (holes) in the conduction (valence) band via two-step optical transitions. An advantage of the extrinsic photoconductive switch is the deeper penetration of the below bandgap light into the switch substrate. Penetration depths of several millimeters to a centimeter or two are possible. This allows switch geometries where the switch contacts are mounted on opposing sides of the switch substrate as shown in FIG. 4. The switch design depicted in FIG. 4 allows for a larger volume of the switch material to carry current and the switch breakdown voltage will be determined by the bulk properties of the switch material. These advantages are offset by the limited number of charge carriers available from extrinsic levels and the requirement that the switch material have mid-band traps to enable two-step optical transitions. Additional doping is required to achieve low “on” resistances (˜1 ohm). The increased doping will create additional energy levels in the bandgap. This increased doping can decrease the switch recovery time by introducing additional deep levels that act as carrier traps. However, the increased doping can increase the switch recovery time if the dopant introduces shallow donor levels that fill existing deep levels with donated electrons. As a result, it is difficult to simultaneously achieve very low “on” resistance and rapid switch recovery.